This article was originally published in the November/December 1995 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.



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Home Energy Magazine Online November/December 1995


Air Sealing in Occupied Homes

by David Keefe

Over the last decade, many new techniques have evolved for improving the airtightness of existing homes. With this increased ability has come the responsibility to be comprehensive.

There are few areas of residential construction that are so commonly misunderstood as air movement within and through houses. While some contractors swear that houses need to breathe and refuse to make any effort to make shells airtight, others are proud of their efforts to reduce uncontrolled air flow but remain unconvinced that controlled ventilation is needed in tight houses. Chances are, both groups are providing work for future retrofitters.

When air sealing was largely ineffective, practitioners generally did little good or harm. Now we can do a great deal of either. The modern energy retrofitter must go beyond simply sealing holes to deal with the ways in which air is supplied, used, and exhausted. The goal is to control, rather than just reduce, the air that flows through our buildings. It often involves the correction of problems more important than high heating bills and includes increasing air flow where appropriate.

Indoor Air Quality

It is well known that building tightness has an effect on indoor air quality. Work that reduces air flow through the building and does nothing else may have unhealthy consequences. However, overall building tightness is not the only, or even the most important, factor in determining the quality of indoor air. Any reasonably tight building requires attention to sources of pollutants, combustion air requirements, and mechanical ventilation.

Pollutant source strength is the most important determinant of indoor air quality. If pollutants are not present in the building, there will be no problem. If the pollutants exist only in small quantities, sensible use of standard ventilation equipment will probably handle them. If moisture or other pollutants are present in very large quantities, the house will very likely have an air quality problem. All of this is true for both tight and loose houses.

Natural ventilation is insufficient to remove common sources of pollutants, including moisture. In mild weather, the air flow supplied by natural ventilation is too small. Regardless of weather conditions, air is not taken from or supplied to the areas where it can be effective. When moisture-laden air leaves the living space, it may cause trouble on the way out. In cold weather, condensation tends to be a problem in bathrooms. All of this is true for both tight and loose houses.

Emissions from unvented space heaters are dangerous. Soil gas can carry massive quantities of pollutants into the house. Furnaces can backdraft. Smokers are a health hazard to their housemates. Irresponsible use of pesticides or chemicals can poison indoor air. All of this is true for both tight and loose houses.

There are some basic strategies that will improve indoor air quality, even with substantial reductions in uncontrolled leakage. The initial analysis of the home should include identifying the sources of any major pollutant, including moisture. It is a high priority to reduce or eliminate these sources. Strategies include both occupant education and hands-on improvements. It is entirely appropriate to refuse to perform air sealing if the sources of pollution cannot be reduced.

Mechanical ventilation in some form is necessary in any tight house (and beneficial in loose ones). Most existing homes without major source problems can maintain reasonable air quality with standard kitchen and bath fans, provided that these are installed sensibly and that the occupants actually use them. No other tradesperson is likely to recognize the need or initiate the action, and most homeowners do not understand the relationship between indoor humidity and cold-weather ventilation. Installing exhaust fans and educating occupants in their use should be part of a comprehensive air management strategy. One sensible approach is to use continuous low-flow fans. These are less occupant-dependent than standard bath fans, and the lower flow rate is less likely to cause undesirable depressurization. There are now reasonably priced fans available that use very little electricity, are rated for constant duty, and are surprisingly quiet (see Bathroom Exhaust Fans, p. 29).

Combustion equipment should be checked for backdrafting potential when it is installed and whenever modifications in its surroundings change air-flow patterns. Although backdrafting is a major concern of air sealers, it depends upon several factors other than overall building tightness. The type of combustion equipment, the strength and location of exhaust devices, interior door closings, and chimney configurations all affect backdrafting potential. Leaving the building leaky and assuming that no hazard exists is not a responsible approach.

High fuel bills, cold drafts, and window condensation are relatively trivial concerns if the indoor air is making people sick. Fortunately, we now know enough to improve rather than degrade the air quality in most homes, and probably enough to identify and avoid most hazardous situations. If the major pollutant sources are eliminated, the combustion equipment is safe, and mechanical ventilation is adequate, the house will usually have good indoor air quality. If any of these requirements is not met, it may not. All of this is true for both tight and loose houses.

Materials for Air Sealing

The criteria for selecting an air sealing material are straightforward. First, it must actually stop air. The frequency with which fiberglass insulation is used as an air barrier points out the need to remember this fact. Second, it must be relatively permanent--ideally, good for the life of the building. Third, it should be safe for both installers and occupants. Preferably, it should be environmentally friendly, although some materials with shortcomings in this respect (such as urethane foam) are so effective that their use is justified.

All other things being equal, a material that does the job faster than other options will improve results and lower cost. There is little sense in economizing on materials. They make up a small portion of the total bill, and long-term results are heavily dependent upon their effectiveness and durability.

One of the most useful materials for an air sealer is urethane foam. Thanks to the recent development of better application tools (foam guns), holes too big to caulk can now be sealed quickly and easily (see Urethane Foams and Air Leakage Control, HE July/Aug '95, p. 25). Many other useful materials are surprisingly inexpensive and low-tech. Air sealing provides an opportunity to use small scraps of material that might otherwise be thrown away. Good results can be obtained with rigid foam insulation, duct board, foil-faced bubble wrap, aluminum flashing, wood, and even cardboard, which is entirely acceptable in locations where it will stay dry. A creative air sealer can make use of materials that were left in the attic because someone thought they had no value.

Over the last decade, many retrofitters have discovered the value of old newspapers (cellulose insulation), especially for holes that are difficult to access. Rather than establish an air barrier at the interior or exterior surface, many retrofitters use the insulation itself as the air barrier. This method, commonly referred to as dense-packing, involves installing cellulose at densities significantly higher than has been standard practice (see Sidewall Insulation and Air Leakage Control, HE Jan/Feb '90, p. 13). Installers are forgoing the one-hole and two-hole methods, where the blower nozzle is inserted into a small hole in the wall, in favor of the tube insertion method. This technique involves somewhat larger holes (one per cavity, regardless of size) and the insertion of a flexible tube all the way to the far end of the cavity. Insulation is forced in at relatively high pressures, and the tube is gradually withdrawn as the cavity packs full. Not only is this a better way to insulate walls, but it effectively seals leaky areas that are difficult to access, including small kneewalls (eyebrows), perimeters of flooring systems, and ceiling cavities.

There are two main advantages to the tube insertion technique. First, the insulation is packed tightly enough so that air cannot move through it under normal atmospheric conditions. This makes it possible to insulate and air seal in one step, and also preserves effective R-value under windy conditions. The air sealing benefits can be impressive. It is not unusual to reduce overall house leakage by 30% to 50% by dense-packing walls and other closed-in cavities. Second, this method ensures total coverage and zero settling. Voids are much less likely to occur, because the material is delivered directly to far corners under high pressure. The recommended procedure involves probing every cavity horizontally and vertically, which is virtually impossible with traditional approaches. Material cannot settle, because it is already packed in at densities significantly higher than the natural settled density of the product. Dense-packing gives consistent R-value, substantial air tightening, and better quality control. Some contractors who use this method guarantee no voids, no bypasses, and no settling.

Pressure Diagnostics

Blower doors offer a way to quantify air sealing results and make it much easier to determine which areas of the house deserve treatment. They work by measuring the relationship between air flow and pressure to determine the size of the holes between indoors and outdoors. This approach can be applied to individual sections of the building to provide more detailed information about the location of the leaks. Pressure checks of individual rooms and ductwork can identify shortcomings in forced-air distribution systems. Pressure pans, used to cover registers, grilles, or other holes temporarily, allow quick checks of ductwork to locate the areas that deserve attention and to verify results. These methods are collectively known as pressure diagnostics (see In Search of the Missing Leak, HE Nov/Dec '92, p. 27 and User-Friendly Pressure Diagnostics, HE Sept/Oct '94, p. 19).

One of the most useful forms of pressure diagnostics involves series leaks. Typical examples include leaks from the living space through attics, kneewalls, basements, attached garages, and other areas to the outside. For example, in an attic, air must pass through both the attic floor and the roof system on its way to outdoors, and the airtightness of each barrier affects the flow. By measuring the pressure difference between the living space and the bordering zone (in this case, the attic) and the difference between the bordering zone and outdoors, one can estimate the relative leakiness of the two barriers. Introducing a specific hole by opening a door or hatch and remeasuring can roughly quantify the leakage in either barrier. One can use this procedure to quantify the leaks in the attic floor, even though they are covered with insulation, and even if the attic is inaccessible. One can then decide how much sealing should be attempted, whether an inaccessible area is worth cutting into, or whether an attempt to seal was successful.

Pressure diagnostics can also be used to determine whether attached garages and other utility spaces are functionally inside or outside the air barrier--an important consideration when these areas contain combustion appliances or cars.


Although a comprehensive approach to air flow in buildings involves much more than fuel savings, a large amount of work is done mainly to reduce the overall leakage of the building. The use of blower doors in a seal-up makes it possible to estimate the savings associated with a given measure. Periodic measurements during the work can indicate whether the sealing that was done since the last measurement was worth the money.

The usual strategy is to seal the big leaks first and save the nitpicking for last. Most projects have a limited budget, so high-priority measures should be done first. In addition, most leaks are actually a series of leaks, and if the air is stopped at the easiest spot, it does not need to be stopped elsewhere. For example, sealing holes in the attic floor will stop leaks through interior partitions or plumbing chases, eliminating the need to caulk cracks in the living space. Dense-packing walls with cellulose will prevent baseboard and trim cracks from leaking. If the high-priority and most cost-effective measures are done first, the project will exhibit diminishing returns as it progresses. Periodic monitoring and establishment of cost-effectiveness guidelines allow crews to determine when the economic cutoff point has been reached.

This monitoring has another benefit. It gives the crew immediate feedback on which measures get good results and which don't. Both their skill and their morale improve as they focus on results instead of tasks. The single most important factor in both the cost and the success of any seal-up is the skill of the crew. Because every job is different and decisions must often be made on the spot, checklist approaches are not effective. It is critical for air sealers to have the knowledge and flexibility to respond to new information as it becomes available.

It is important to remember that some things are worth doing for reasons of health and safety rather than economics. Giving these items high priority may mean that some cost-effective measures cannot be undertaken.

Developing an Air Sealing Strategy

The caulking and weatherstripping that formed the basis of traditional air sealing are used much less extensively today, as workers become more skilled at finding better options. Caulking is mostly limited to areas of high moisture or exfiltration or both, where it is used to control condensation and infiltration that causes comfort problems. Routine weatherstripping of doors and windows is no longer standard practice. The reason is simple: More air can be stopped elsewhere for less money (and longer lifetime). Another consideration is that air moving through windows and doors does less harm than air that travels through cavities, depositing condensation on the way.

The best opportunities for reducing overall leakage are often found at the top and the bottom of the house. These areas experience pressures from warm air rising. They also tend to have problems other than heat loss. Leaks at the top of the building, where air usually goes out, often cause condensation problems. Leaks at the bottom can carry moisture, radon, or whatever else is in the soil gas, into the house. Another reason to look at the top and bottom is that the rough areas bordering the living space tend to have large holes that can be fixed relatively cheaply.

Sealing at the Top

One of the most important leakage sites is the attic floor. There are some building sections where absolute tightness may not be desirable, but this isn't one of them. Any leak in the attic floor that can be sealed safely and cost-effectively should be. Standard locations include plumbing vent stacks, electrical penetrations, chases around chimneys, open tops of interior partitions, and gaps around the penetrations for mechanicals (ductwork, recessed lights, bath fans, and so forth). Areas exposed to high humidity (bathrooms) deserve special attention. The attic access hatch is often loose; it may be more important to weatherstrip this hatch than any door or window in the house.

Standard recessed lights are a real headache for air sealers. They are designed to be cooled by exfiltrating air and cannot be sealed or insulated without danger of overheating. The best approach is to replace them with surface-mounted or Type IC fixtures (these can be safely covered by insulation). Failing that, about all that can be done is to seal the gap between the fixture and the rough opening in the ceiling.

Bathroom exhaust fans often deliver air to the attic rather than outdoors. Many contractors seem to think that this is fine because the attic is vented. Don't believe it! All bathroom and kitchen exhaust fans must be ducted to the outside.

Sealing at the Bottom

In the basement, other rough holes exist. Some of them are below grade. It is often assumed that they can't leak because they are covered with dirt--often several feet of it. But they can leak--and do. When they do, they bring in not plain air, but soil gas. At the very least, this air is wet. At worst, it can contain radon, sewage gases, or pesticides.

Above-grade holes in the basement perimeter can include penetrations for utilities, gaps at the top of the foundation walls or sill, or perhaps windows in disrepair. Dry-laid stone foundations leak like crazy everywhere. There are probably leaks between the basement and the living space. Whether to seal the perimeter, the ceiling, or neither depends upon the circumstances. The main variable is the presence of combustion equipment in the basement and the locations of exhaust appliances (such as clothes dryers and exhaust fans). It is important to allow sufficient air for flues and to prevent exhaust appliances from depressurizing the space where the heating plant is. All air sealers should know and perform combustion safety tests for the potential of backdrafting (see Combustion Safety Checks: How Not to Kill Your Clients, HE Mar/Apr '95, p. 19).

It is important to pay attention to areas that are hard to access. The unpleasantness of dirty, cramped spaces is one reason why significant leaks are likely there--whoever last worked in those areas was probably not too concerned with being meticulous; he or she just wanted to get out quickly. And while Joe Homeowner may have stuffed a few rags around easily accessible windows, he probably hasn't visited the cat hole in the far corner of the crawlspace. The good news is that a worker with a healthy attitude can get good job satisfaction by attacking such a challenge. It usually turns out not to be as bad as imagined, often gets impressive results, and helps to foster a can do attitude that helps elsewhere. Getting dirty (really dirty) is a fact of life for effective air sealers.

Figure 1 Figure 1. Air leakage pathways through a kneewall. Even an insulated kneewall space often allows significant air leakage. In effect, the interior floor of the second story can be directly ventilated with outdoor air.

Kneewalls and Suspended Ceilings

Kneewall spaces (attic areas outside of short top-floor walls) are particularly troublesome. These spaces often connect with cavities between floors to admit air from all over the house (including air leaking through interior walls from plumbing and electrical penetrations). If there is insulation in the roof slope above the kneewall space, it often lacks an interior air barrier and therefore performs badly. Kneewall spaces generally connect with the main attic. If insulation exists on the kneewall itself, it is usually incomplete and sloppily installed. There are often leaky access doors.

In most kneewall spaces, and in many other utility or unheated areas, the main problem is that no one has made a conscious decision as to the location of the thermal envelope (see Beauty and the Beast Upstairs, HE Mar/Apr '95, p. 27). Spaces like these should be designated inside or outside. For kneewalls specifically, including them in the heated space often makes sense, especially if they are being used for storage. It means more volume of useful space for less surface area of heat loss, fewer corners and details, and hatches that do not need to be treated or kept closed. Other areas with similar problems include chases for plumbing, built-in cabinets and drawers, and kitchen soffits.

In commercial buildings, suspended ceilings (removable panels that rest on a grid hanging from the framing above) are often used to allow easy access to ceiling-hung utilities. In residences, they seem to be used to hide the old deteriorating plaster ceiling. Sometimes fiberglass insulation has been laid on top, adding insult to injury. Somewhere there needs to be an air barrier, and it should be below most of the insulation. In practice, this probably means at the bottom of the ceiling joists, where the ceiling was, or should have been, originally. Auditors and retrofitters should develop the habit of looking above every residential suspended ceiling. After all, there must be some sort of problem up there; otherwise, why would they have covered it up?


Fireplaces are big holes. Air almost always moves up and out, so the cold draft is felt elsewhere. Often fireplace dampers are open for long periods without anyone knowing. If the damper doesn't close well, or if the fireplace is seldom used, some sort of airtight covering is in order. Glass doors can work, if they are reasonably tight (most aren't). A plywood door can be made and need not look too ugly. This is most appropriate for seldom-used fireplaces where an airtight seal can be obtained (this is tough to do on rough stone or brick). Other options include dampers for the top of the chimney and inflatable plugs that fit up the chimney out of sight. Practitioners should explain to occupants the importance of keeping the damper shut when the fireplace is not in use, and the general inefficiency of most fireplaces. They should also be aware that smoldering fireplaces produce carbon monoxide and backdraft easily.

A Question of Balance

Forced-air heating systems can have a large effect on the overall leakage of the house. They can also create moisture, comfort, and safety problems. Sealing ducts to make them airtight can improve the efficiency of forced air systems significantly. In addition, unbalanced systems will deliver air to one part of the building while returning it from another. This can create quite powerful pressure differences--both between parts of the building and between indoors and outdoors. For example, a single-return system may deliver air to bedrooms with closed doors, pressurizing these rooms while depressurizing the central part of the house. This results in inefficient delivery of heat and increased air flow to and from outdoors.

Another common pressure problem can arise when a basement furnace has a leaky return system in the basement. The furnace delivers most of the heated air to the living space but takes a significant portion of the return air from the basement. This pressurizes the living space and depressurizes the basement. The powerful furnace fan can depressurize the basement relative to outdoors enough to backdraft the furnace or a water heater in the same space.

Although all this is old news for competent commercial HVAC contractors, most residential heating and cooling technicians do not understand these phenomena and believe that leaks and imbalances in forced air systems are relatively unimportant. They are wrong. Like many aspects of air flow in houses, duct leakage and imbalances have usually been ignored by all of the previous tradespeople who have worked on a house. They either did not understand the issues involved or did not consider them part of their job. Much of the appropriate retrofit work involves doing things that were missed or were done improperly by others.

An Air-Sealer's Obligation

We have both the ability and the obligation to look at air flow not as an uncontrolled phenomenon to be avoided, but as a critical and essential function to be managed. A comprehensive approach that addresses the issues raised in this article will improve comfort, reduce heating bills, and (more importantly) make the indoor air healthier and safer.

Note: A version of this article was originally published in Building Solutions: Proceedings of the Joint Conference of the Energy-Efficient Building Association and the Northeast Sustainable Energy Association, Vol. 1, A41-A56, Wausau, WI: EEBA/NESEA, 1993.

David Keefe, president of Building Tune-Ups Inc, is a consultant from Fairfax, Vermont.


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